Setting Peer Port and Web UI with Docker Compose

[terdinglage]

I am trying to set a different Web UI and a fixed Peer Port using a Docker Compose file with the following entries:

  TRANSMISSION_WEB_UI: combustion
  TRANSMISSION_PEER_PORT: 54321

Is this not the correct syntax? Nothing changes with my setup after adding these lines and restarting. I am able to manually enter the desired peer port, but it gets overwritten after a while.